Methods that correspond to the above definition are already known. In particular, the article by A. B. Frazier et al. entitled “Metallic Microstructures Fabricated Using Photosensitive Polyimide Electroplating Molds” published in the Journal of Microelectromechanical Systems (Vol. 2, N deg. 2, June 1993) describes a method of fabricating multi-level metallic structures by galvanic growth in polyimide moulds made by the photolithography of photosensitive resin layers. This method includes the following steps:                creating a sacrificial metallic layer and a priming layer on a substrate for subsequent galvanic growth,        spreading out a layer of photosensitive polyimide,        irradiating the polyimide layer with UV radiation through a mask that matches the contour of one level of the structure to be formed,        developing the polyimide layer by dissolving the non-irradiated parts so as to obtain a polyimide mould,        filling the mould with nickel to the height of the mould by galvanic growth, and obtaining an approximately flat top surface,        depositing a thin layer of chromium over the entire top surface by vacuum plating,        depositing a new layer of photosensitive resin on the chromium layer,        irradiating the resin layer through a new mask that matches the contour of the next level of the structure to be obtained,        developing the polyimide layer so as to obtain a new mould,        filling the new mould with nickel to the height of the mould by galvanic growth,        separating the multi-level structure and the polyimide mould from the sacrificial layer and the substrate,        separating the multi-level structure from the polyimide mould.        
It will be clear that the method that has just been described may, in theory, be repeatedly implemented to obtain metallic structures that have more than two levels. One drawback of this method is the necessity of obtaining an approximately flat top surface during each galvanic deposition step. One solution to this problem is disclosed in EP Patent No. 1,835,050, which teaches that it is possible to obtain the desired flat surface by machining the metallic structure and the mould in situ after the galvanic deposition step. However, the presence of in a clean room machining installations could potentially create more problems than it solves. The alternative, which consists in making return trips between a machining installation and the clean room, is also incompatible with the extreme cleanliness generally necessary for photolithography and galvanic deposition.
EP Patent No. 0,851,295 discloses another method for fabricating multi-level metallic structures in moulds made by UV photolithography. This second method includes the following steps:                a) creating a sacrificial metallic layer on a substrate,        b) spreading out a layer of photosensitive epoxy resin with a thickness of between 150 and 700 microns,        c) pre-baking said layer at a temperature between 90° and 95° for a duration that depends upon the deposited thickness,        d) irradiating the resin layer with UV radiation through a mask that matches the contour of one level of the structure to be obtained,        e) post-baking said layer to cause polymerisation        f) reproducing steps b) to e) at least once, and if required, depending upon the desired contour for structuring the new layer of photoresist, using a different mask for step d),        g) developing the superposed resin layers by dissolving the non-irradiated parts so as to obtain an epoxy resin mould,        ii) forming a primary metallisation over the entire surface of the mould, then completely covering the mould by galvanic depositions so as to form the multi-level metallic structure,        h) separating the substrate from the assembly formed by the epoxy resin mould and the metallic structure,        iii) separating the multi-level structure from the resin mould.        
One advantage of this second method is that the resin mould is completely made prior to the galvanic deposition step. Any machining of the multi-level metallic structure can thus be performed afterwards, outside the clean room, without any risk of contaminating the latter. However, this second method also has some drawbacks. In particular, the superposed resin layers are developed during the same step. In these conditions, the prebake step of the second photoresist layer necessarily also constitutes a post bake of the first layer. It is thus very difficult to adapt the temperature and the duration of the bakes in an optimum manner.